Doesn't seem like it would be too hard to do in a film if they knew what they were doing,
>1. Establish Apokolips, the New Gods, Steppenwolf, and through Steppenwolf, Darkseid
You don't "Have" to establish the New Gods, in a similar way that you don't "Have" to establish atlantis to enjoy an Aquaman story. You could have it set in the past with a vague elder type character explaining that there was a war, a war with a far away army led by steppenwolf, General to the army of Darkseid, showing Darkseid in the shadows, no need to fully reveal him right now, maybe standing on a cliff on apokalypse with red smoke coming from his eyes highlighting him. Or you could have something similar to the story of Krypton's past from MoS, quick bit of exposition giving you everything you need to be in the know.
>2. Show how humans, Amazons, and Atlanteans defeated Darkseid.
Mother boxes or some shit, maybe they got a weapon from New Genesis? But that would open up a whole new can of worms
>3. Explain the motherboxes.
Sentient computers that can do loads of super cool shit, not that difficult to explain, maybe have someone from star labs or cadmus explain it or some shit
>4. The resurrection of Superman, and how it's affected him.
No clue how they will do this one, i could imagine any character development they give him will be done after the final battle with steppenwolf, when he has a conversation with batman saying something like "Facing my own mortality has shown me how much more the world needs me, It's time to embrace who I really am and become the man my father (Jor-El) wanted me to be" Or some shit, sounds bad, but i'm not a writer
(1/?)